Polymer films as of polyethylene or nylon, particularly those having an extremely small thickness of 50.mu. or less, are generally factory-produced continuously at high speeds. It is highly important for product quality control and material savings that the thickness of the film as it is manufactured be measured and monitored to see if it meets required specifications. Film thickness measurements have conventionally been carried out by .beta.-ray film thickness gages of the contactless type. However, since the .beta.-ray film thickness gages use a radioisotope as the source of .beta.-ray radiation, they are dangerous to handle. Furthermore, a radiation shield, which happens to be quite expensive, must be installed for protection, and under some existing laws a chief radiation technician must operate or monitor the gage. Therefore .beta.ray film thickness gages have been employed on only a limited basis.
As an alternative to the .beta.-ray film thickness gages, contactless-type infrared film thickness gages which rely on infrared radiation as a measuring medium have been employed. Infrared film thickness gages operate on the principle that infrared radiation has different attenuation coefficients for different wavelength bands for a film irradiated by the infrared radiation. More specifically, a test film is irradiated alternately with an infrared ray having a wavelength .lambda.R (hereinafter referred to as the "reference wavelength") which has a smaller attenuation coefficient, and an infrared ray having a wavelength .lambda.M (hereinafter referred to as the "measurement wavelength") which has a larger attenuation coefficient. A light detector detects the intensity of light after it has been transmitted through the film, and the detected light intensity for the two wavelengths is converted into a common logarithm ratio to calculate the thickness of the film. However, due to sudden changes in the amount of background light caused by energizing a fluorescent light or opening a window shade or blind, measurement errors occur. These measurement errors often exceed the acceptable tolerance level required for measurements of this type. Therefore, an acceptable measuring device having the necessary accuracy even when subjected to background sources is needed.
As mentioned above, the present invention relates principally to an infrared film thickness gage capable of continuous on-line measurement of the thickness of plastic film being produced by a film blowing process (also known as an inflation method).
Film blowing for forming plastic film is generally carried out by an extruder which melts and compresses resin material and extrudes the melted mass through a ring die having an annular gap to form a plastic film into a tubular form. The ring die has an air blowing pipe for introducing air into the interior of the tube to inflate the tube.
For making the quality or thickness of the film more uniform, the apparatus for forming plastic film measures the thickness at various parts of the film at a position where the plastic film is inflated into the tubular form, and adjusts the gap in the ring die to keep the film thickness uniform.
Thickness gages for measuring the thickness of an inflated film tube must measure films having a thickness ranging from about 0 to 300.mu., at a precision of .+-.1.mu.. To meet these requirements, an infrared film thickness has been employed to measure the thickness of inflated film tubes. The infrared film thickness gage operates on the principle that infrared radiation has different attenuation coefficients for different wavelength bands for a film irradiated by the infrared radiation. More specifically, a test film is irradiated alternately with an infrared ray having reference wavelength .lambda.R which has a smaller attenuation coefficient, and an infrared ray having a wavelength .lambda.M which has a greater attenuation coefficient. A light detector detects the intensity of light after it has been transmitted through the film and the detected light intensity for the two wavelengths is converted into a common logarithm ratio to calculate the thickness of the film.
When measuring the thickness of film made during the inflation process the interior of the inflated tube tends to be heated up to a temperature of about 200 degrees Celsius. Therefore, a light source including a filter and motor and installed inside of the film tube is exposed to extreme heat and has a relatively short service life and poor operation stability. In an attempt to overcome these problems a conventional infrared film thickness gage arrangement has been used wherein only the source of infrared radiation is positioned within the film tube. The photodetector is installed outside of the film tube to pick up infrared rays of the measurement and reference wavelengths.
In such an arrangement, a tungsten-filament lamp has been used as the source of infrared radiation. However, since the direction distribution of radiation intensity of a tungsten lamp is not uniform, upon scanning the zero point shifts as will be explained.
Infrared detectors are based on the principle that the resistance of a photosensitive element varies in proportion to light intensity to detect an infrared ray. The infrared detectors have resistance vs. light intensity characteristics which are different for different wavelengths. While the resistance vs. light intensity characteristics of the infrared detectors have a relatively rectilinear relationship for various wavelengths in a region in which the light intensity is small, the characteristics become nonlinear as the light intensity increases, and differ from each other with the wavelengths. Where the directional distribution of radiation intensity of the infrared radiation source is nonuniform as described above, the zero point shifts by an amount of about 1.mu. to 1.5.mu. in a scanning operation. Thickness gages using tungsten-filament lamps have an accuracy far less than 1.mu., a level required by thickness gages for inflated tubular films. Therefore, such thickness gages are useless for measuring films made during the inflation process.